Bonding to Existing Concrete

posted by Bob

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.

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745 USER COMMENTS

I am looking to add a 1" layer of rigid foam over an existing concrete floor to allow for radiant floor tubing. What is the minimum thickness pour needed to cover my tubing and allow for ceramic tile floor?
- Richard
Tuesday, February 21, 2017 at 2:07 PM

Terry, we do not have a product that can be used over the Schluter materials. You will need to get a product recommendation from Schluter.
- Lee-Technical Service
Monday, February 20, 2017 at 11:03 AM

An existing footing had a 4"x 4" cedar post to help support a 8' deck.That post has rotted out leaving only a hole. Replacing the footing entirely is prohibitive. Any suggestions regarding refilling the hole with a sacrete product?
- James
Monday, February 20, 2017 at 9:53 AM

Hi! Would Sakrete scratch coat work over Schluter Becotec Thermo for infloor hot water piping system? This product is a polyurathane foam that is an inch and 3/8 thick floating over an existing concrete floor which will have to be filled and leveled over top (1 & 1/2" to 2" thick of Sakrete). Thanks, Terry
- Terry
Sunday, February 12, 2017 at 8:13 AM

Eldon, you can use the Flo-Coat Concrete Resurfacer to get a fairly smooth finish. This material is comparable to the Top-N-Bond except for the fact that the Flo-Coat is a squeegee applied material. It is a flowable product.
- Lee-Technical Service
Tuesday, January 31, 2017 at 9:27 AM

Steve, Lee can chime in if this advice is bad, but one thing I've come across is the idea of doing this repair in multiple stages: 1) You set up a 1" screed guide (like a piece of wood) fill in the deep part with stone mix 2) you set up a 1/2" screed guide and fill in the next part with sand mix 3) you use something like top n bond to feather in the final part of the fix Alternatively, you could start using an SLC at step 2), which are often good to 1" thk.
- Eldon
Monday, January 30, 2017 at 10:48 AM

My basement floor (350 sq ft) slopes away from the drains slightly, so I'd like to build it up with a slope in the right direction. I think I can use top n bond in this application, but I'm anticipating a pretty rough/uneven surface when I'm done. Do you sell another product, like a semi-self leveling compound, that I can lay down in about an 1/8" thick layer to smooth it out after I've trowelled the top n bond as well as I can? Remember that I'm not looking for a level floor, so much as a smooth one. Thank you!
- Eldon
Monday, January 30, 2017 at 10:32 AM

Mike, to replace the cultured stone and the joints, you will need to use the Stone Veneer Mortar. Keep in mind that as longs as the vehicles continue to drive on it, then it will continue to be a maintenance issue.
- Lee-Technical Service
Monday, January 30, 2017 at 10:16 AM

Our neighborhood has a median at the entrance. There is a concrete ribbon that is 2' wide and 240' in circumference that has vertical trowels on it. Covering the ribbon (in most places) is cultured stone and mortar. This sits only a few inches off the ground and has been damaged by heavy trucks driving on it. What's the best product to do patchwork repair on the joints, including replacing some of the dislodged stones?
- Mike
Sunday, January 29, 2017 at 2:24 PM

Steve, unfortunately they are correct. There are very few options. You could consults a company with a slab jack product, which works very well. They will drill a series of holes and pump an expanding material in. This will lift the slab. The steps will be repeated until the slab is back into position. The only other option is their way.
- Lee-Technical Service
Tuesday, January 24, 2017 at 12:45 PM

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